Multiple Lithium-Ion Battery Anodes from One Silicon Wafer

The extraction of multiple high performance anodes from a single silicon wafer for lithium-ion batteries was recently demonstrated by researchers at Rice University and Lockheed Martin.

The procedure uses simple silicon to replace graphite as the anode in rechargeable lithium-ion batteries. In a battery, the anode is the negative electrode from which electrons flow out from the electrolyte towards the device being powered. The end result of this research could be much longer-lasting, more powerful batteries for use in commercial electronics like smartphones, camcorders and even electrically powered vehicles.

The research, led by Sibani Lisa Biswal, assistant professor of chemical and biomolecular engineering at Rice University, was reported in the journal Chemistry of Materials recently. A Swiss cheese-like silicon sponge is electrochemically lifted off from a wafer in the new process. The “sponges” are able to store more than four times their weight in lithium.

Silicon in Lithium-Ion Batteries

Silicon, as one of the most abundant elements on Earth, is a top candidate to replace graphite in the anode portion of batteries. Other ways to use silicon in lithium-ion batteries are being researched currently, including silicon nanotubes or nanospheres (at Georgia Institute of Technology), silicon nanopowder in conductive binders (Lawrence Berkley National Laboratory), silicon/titanium dioxide composite nanowires (University of Maryland), and silicon whiskers (Physical Sciences Inc).

Silicon has the property of expanding as it absorbs lithium ions. The researchers took advantage of this property by designing sponge-like configurations of the silicon. With micron wide pores, 12 microns deep, room is allowed for internal expansion when soaking in lithium ions, without degradation of battery performance.

An earlier advance by Biswal and her team found porous silicon was able to soak up 10 times more lithium than graphite.

Biswal, Madhuri Thakur, a Rice research scientist, colleague Michael Wong, professor of chemical and biomolecular engineering, and Steven Sinsabaugh, Lockheed Martin Fellow, first disclosed the promise that these silicon sponges hold for batteries in 2010 at Rice’s Buckyball Discovery Conference.

Thakur even then had seen opportunities for further development, since the solid silicon substrate served no purpose in absorbing lithium.

In their new work, they developed a way for the electrochemical etching method used to create the pores to also separate the sponge from the substrate. The substrate can then be reused to make more sponges. The team observed that a minimum of four films can be drawn from a single standard 250-micron-thick silicon wafer. Additionally, removing the sponge from the silicon substrate eliminates a limiting factor to the amount of lithium that can be stored.

Enhancing Anode Conductivity

The researchers also established a way to make the pores 50 microns deep. Once lifted from the wafer, the silicon sponges, now open at the top and bottom, were enhanced for conductivity through soaking in a conductive polymer binder known as pyrolyzed polyacrylonitrile.

The result is a robust film that can be attached to a current collector- the researchers used a thin layer of titanium on copper- and placed in a battery arrangement. The upshot was a working lithium-ion battery having a discharge capacity of 1,260 milliamp-hours per gram. This capability should lead to batteries that last longer between charges.

A comparison of batteries using the silicon-lithium film before and after the PAN bake treatment was done during the research. Before treatment, batteries started with a discharge capacity of 757 milliamp-hours per gram, dropped rapidly after the second charge-discharge cycle and failed completely by cycle 15. Batteries using the film after soaking in pyrolyzed polyacrylonitrile increased in discharge capacity over the first four cycles, which is typical for porous silicon, according to the researchers, and the discharge capacity remained consistent through 20 cycles.

Slated for further investigation by the researchers are techniques which hold the promise to vastly increase the number of charge-discharge cycles. This would be a vital feature for commercial applications, where rechargeable batteries are typically expected to last for years.